Skeletal Muscle Mitochondria: The Aerobic Gate?

Abstract

At an animal’s maximum aerobic capacity ($\rm\dot{v}$O₂max), the O₂ flowing through the respiratory system is consumed by a functionally exclusive sink, skeletal muscle mitochondria. Thus, O₂ consumption will never exceed the muscles demand. If the system is ideally designed, structures upstream to the skeletal muscle O₂ sink must be built to insure adequate O₂ delivery to the working muscle. There are a number of structure-function solutions available to supply the demanded to the muscle; these have been found to vary, often ontogenetically, with hypoxia, training, etc. But there is one relationship that is invariant: Total O₂ uptake can be predicted by the total (active) skeletal muscle mitochondrial volume. In aerobic and sedentary animals, across a range of body sizes, maximum (in vivo) mitochondrial O₂ consumption is constant among mammals (at approximately 2000 O₂ molecules per square micron of inner mitochondrial membrane per second). Because the volume of mitochondria is one of the most plastic of all respiratory structures, we interpret this relationship as suggesting that skeletal muscle mitochondria alone sets the demand for O₂ and, thus, the volume of skeletal muscle mitochondria dictates an animal’s maximum aeorbic capacity.